Minimum pressure to a txv

Low ambient control needed

Do TXVs have to have a minimum pressure at the inlet in order to work as designed?
I understand by lowering head pressure then efficiency is increased and that capacity would be decreased but how low is too low a head pressure for the valve to work as designed and not cause problems.

In the winter times I see so many units especially the air handler types with internal and external units that are running at a head pressure of 10-11 bar( fixed speed compressors)
just basic units but because the indoor fan is a fixed speed fan it just runs at a constant speed and can't regulate the HP

The evap coils are always freezing up even *though the return air from the building onto the coil is usually 21-22degrees.

Gas charges are ok subcooling is around 6-8k and superheat is always being controlled at 6k and no moisture present in the system, other than the coils freezing up the systems heat and cool fine.

Other than the humidity factor all I can think is the low head pressure could be affecting the txvs but not too sure.
Can any one shed any light on this for me please.

I have the exact same problem on my AHU. Both are in a food production area and my initial thought of the freezing evap was when they wash down between product changes the steam created would be drawn across the coil and cause it to freeze. However after monitoring it i am satisfied it wasn't the cause.

I am also suffering with low head pressure due to the weather in the UK right now

take a look at this guys http://www.hvacfun.com/a-everything-...-about-txv.htm

Hi Marc.
Yours is a good post especially as you point out at this time of year the lower head pressures become an issue.
I completely concur!
In my experience if the discharge pressure is dropped (For what ever reason) too much.
Some styles of expansion valve close down or more commonly don't open fast enough too prevent a LP shutdown.

Some manufacturers are now starting to have software settings to accommodate Low ambient conditions and fan speed control.

We have a site where we cover part of the condenser to accommodate the lower ambients experienced.
That in itself is another story.

But as is your original question, yes you do need a certain amount of "lift" across the Exp valve.
For it to work correctly.
Some of the earlier Danfoss valves on adapkool needed something like 100psi diff for them to operate correctly.

You will find it difficult to find this written down anywhere though!

Good Luck Grizzly

Quote:

---

Originally Posted by Grizzly

We have a site where we cover part of the condenser to accommodate the lower ambients experienced.
That in itself is another story.

---

We have exactly the same problem on a few of our sites as well and have resorted in blocking the condenser like you described.
I've worked on an Airedale condenser recently that seemed to regulate the HP by use of the evaporator fan so was thinking that is how some units work in the winter but i'm still not sure about that.

Hi Marc.
Some of the sites I look after have tier chillers running all year round despite the ambient being well below their design parameters.
Extreme I know but as they say "That's Life"!
I would be interested to know which Airedale that was as they have 2 different selectable control strategies.
Add this to their free cooling units and it can get interesting.
No disrespect to Airedale, theirs is some good kit, Only even theirs don't like all ambient conditions.

Grizzly

Sporlan always recommended a liquid line temperature of 100°F, 38°C, at the TEV inlet. They also provided balanced port valves for floating head systems.

http://www.sporlanonline.com/10-9_32011.pdf

There are lots of ways around the problem as long as there is enough money. ;)

Hi Mark.
a txv is a pressure related variable control devise, designed to deliver a variable volume of liquid at a varaible pressure differential between high and low sides. Change one variable and flow drops or increases. Low ambients is the killer.

This is more complicated than it first looks, firstly by dropping your head pressure, you increase the possible net cooling effect, which mean that the TD across the coil will increase to meet "equilibrium" (lower SST) also you are likely to find that the air on has a lower RH (fresh air make up has lower amount of humidity), this will also increase TD.
If your maintaining a constant suction superheat, then the valve is controlling so is doing what it should and as yet is not being effected by the lower liquid pressure. When you loose control your SH (increasing superheat), then you do not have enough pressure to drive the correct amount refrigerant through the valve.
You could look at reducing your superheat setting, to make greater use of your coil, this should lift the SST slightly.
Just one thing is the superheat constant even with a coil free of ice!

Quote:

---

Originally Posted by Grizzly

Hi Marc.
Some of the sites I look after have tier chillers running all year round despite the ambient being well below their design parameters.
Extreme I know but as they say "That's Life"!
I would be interested to know which Airedale that was as they have 2 different selectable control strategies.
Add this to their free cooling units and it can get interesting.
No disrespect to Airedale, theirs is some good kit, Only even theirs don't like all ambient conditions.

Grizzly

---

Hi Grizzly,
Tier units? I'm not familiar with those, I take it they are heat pump chillers?

I think the Airedale units were CUHS 6.0, the ones where the fan discharges the air upwards.

I noticed when it was in heat mode and the outdoor fan speed controller was in auto that it the fan as
The fan speed increased suction pressure increased but also discharge pressure increased and the
Opposite if the fan slowed down.
I would have expected the fan to run flat out in order to evaporate all the refrigerant in the coil but
It didn't- it looked like it was maintaining head pressure.

Quote:

---

Originally Posted by Brian_UK

Sporlan always recommended a liquid line temperature of 100°F, 38°C, at the TEV inlet. They also provided balanced port valves for floating head systems.

http://www.sporlanonline.com/10-9_32011.pdf

There are lots of ways around the problem as long as there is enough money. ;)

---

Hi Brian, thanks for the link, I'll have a read through that later.

As you've said I always thought that minimum temp to a TXV should be 38-42C but I couldn't find any
References to this when I've been searching.

Quote:

---

Originally Posted by Magoo

Hi Mark.
a txv is a pressure related variable control devise, designed to deliver a variable volume of liquid at a varaible pressure differential between high and low sides. Change one variable and flow drops or increases. Low ambients is the killer.

---

If low ambients cause so many problems then why don't the manufacturers design their systems to
Combat this.
On a few systems I've worked on they have electric heaters which on the trend are labeled as frost heaters. I always set these to come on at 10C which is the maximum I can set them to but to be honest they don't increase the head pressure that much.

Quote:

---

Originally Posted by mad fridgie

This is more complicated than it first looks, firstly by dropping your head pressure, you increase the possible net cooling effect, which mean that the TD across the coil will increase to meet "equilibrium" (lower SST) also you are likely to find that the air on has a lower RH (fresh air make up has lower amount of humidity), this will also increase TD.
If your maintaining a constant suction superheat, then the valve is controlling so is doing what it should and as yet is not being effected by the lower liquid pressure. When you loose control your SH (increasing superheat), then you do not have enough pressure to drive the correct amount refrigerant through the valve.
You could look at reducing your superheat setting, to make greater use of your coil, this should lift the SST slightly.
Just one thing is the superheat constant even with a coil free of ice!

---

Yes I suppose i could try to open the valve slightly to get the SST above 0DegC.
Not all the systems I work on have accumulators though so I wouldn't be able to try this approach on some systems.
On the AHU types I work on, the manufacturers have been back to fit defrost stats to switch the 4 way valves when the coils begin to I've up but to me this is masking over a problem i.e why the coils are freezing up in the first place.

Yes the superheat seems constant when the coil is free of ice, why do you ask?

If superheat is constant across the whole range, then your pressure is not effecting the TEV performance.
The coils are freezing because you have more duty from from your refrigeration and less load, (lower moisture removal) The combination will will allow the suction to drop well below 0C, frost will build, whence ice is built the suction just continues to fall making more and more ice. Id SST were to remain the same, you would get around 15-20% more cooling, but your indoor coil is the limiting factor, so evap temp must drop, this does cause balance out some of the gains made, by dropping the head pressure, so you could be getting around 8% actual increase. You need to check your dew point of the air entering the indoor, I suspect this lower than design.
If this is a major, you could look at fan speed, or sporlan head control system or just a simple hot gas injection to keep the evap above freezing.

Thanks MF, that makes perfect sense.
Next time I'm at one of these units I'll take some better readings including dew point temp and see if that is the case, I'll also keep a closer eye on the SH to see if it stable.